Air-kerma modulation effects on the energy spectrum of a 137CS irradiator using Monte-Carlo techniques

Abstract

Calibration of radiation protection instrumentation such as survey meters is typically performed using 137Cs irradiators. During calibration, various thicknesses of lead attenuators are added and the source-to-detector distance is adjusted to modulate air-kerma rate. Variations in the energy spectrum as a result of these modulation techniques are unknown, and may affect the response of energy-dependent survey meters during calibration. The purpose of this work was to characterize the spectral changes that result from modulation of the air-kerma rate for the Hopewell Designs G-10 dual source irradiator at the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL). An experimentally validated Monte-Carlo model of the irradiator geometry was used to investigate energy spectra variations as a function of attenuation and distance. Lead attenuator thicknesses of (0, 0.64, 1.22, 2.22, 4.32, and 5.54) cm were used in this study. The irradiator geometry was modeled using MCNP6 Monte Carlo (MC) transport code and validated by comparing simulated and measured percent depth dose (PDD) curves and cross-field profiles. Spectra were tallied at (100, 200, 300, and 400) cm from the source. The effects of room scatter on the energy spectrum were also investigated. Simulated and measured PDDs agreed within the associated uncertainty and profiles agreed within 3% or 3 mm distance to agreement. The mean energy for simulated spectra at 100 cm from the source varied from 593 keV to 654 keV as attenuation increased from no attenuation to 5.54 cm of lead. The mean energy of the spectra remained consistent at distances ranging from 100 cm to 400 cm for all attenuation thicknesses and for both geometries in which the vault walls were defined and undefined. This study used MC methods to show that the modulation of air-kerma rate using attenuation and source-detector distance significantly changes the energy spectrum. This knowledge may improve the accuracy of any calibrations using this source.